The (meth)acrylate syrup has been used as an intermediate raw-material for methacryl resin substrate, optical materials including optical transmission fiber or optical waveguide, acryl artificial marble, floor materials, adhesives, and medical supplies.
The (meth)acrylate syrup can be produced by bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization and photo polymerization owing to the excellent reactivity of (meth)acrylate monomers. As these materials for use in the respective applications became highly functional, the use of solution polymerization, emulsion polymerization and suspension polymerization for their production required much energy for the removal of residues. In addition, in the use of these polymerization processes, it was difficult to exhibit high function, and the load to environment was high. For these reasons, it is a tendency to produce (meth)acrylate by bulk polymerization or photo polymerization which is carried out in the absence of solvents.
In carrying out bulk polymerization or photo polymerization, heat transfer is difficult due to the absence of a solvent, and in particular in carrying out the process in a general batch reactor, the reactor temperature is difficult to control, resulting in a high possibility for reaction runaway, and the termination of production of radicals is reduced due to a rapid increase in viscosity resulting from an increase in conversion rate. Thus, a phenomenon, such as the partial formation of gel, occurs, and non-uniform resin with wide range of molecular weight is liable to be, obtained.
To overcome the problems of the high chance of reaction runaway and viscosity increase, the use of a semi-batch, continuous or plug flow reactor in place of the batch reactor has been reported. Japanese patent laid-open publication Nos. Sho 40-003701, Hei 11-255828 and 2000-159816 disclose high-temperature polymerization with the use of this continuous polymerization method.
However, the polymerization in this reactor has a problem in that it has a high economic burden, because not only the reactor itself is expensive but also the cost of utilities is significant. Another problem is that it is unfavorable for the production of small amounts of many different species, although it is suitable for the production of large amounts of a few species.
In an attempt to solve these problems, a method is known that carries out polymerization in the batch reactor under the mildest possible conditions. This is a method where the temperature of a reaction system is maintained at a constant level while forcibly terminating polymerization when the conversion or viscosity of the reaction system reaches a given level. Regarding the method for the termination of polymerization, Japanese patent laid-open publication No. Hei 1-11652 discloses the termination of polymerization by the addition of a polymerization inhibitor, and Japanese patent laid-open publication No. Hei 9-67495 discloses polymerization termination, such as quenching by the addition of monomers.
However, this polymerization method cannot become a fundamental solution because it has shortcomings in that it causes a great increase in viscosity at the latter half of reaction, shows a difference in physical properties depending on the time point of completion of reaction, and results in polymer syrup with poor storage stability because a polymerization initiator remains in the syrup.
In an attempt to solve these problems, there have been many solutions where a batch reactor is used but reaction runaway does not occur and the molecular weight of syrup is easily controlled.
First, there are examples where bulk polymerization was carried out without causing reaction runaway due to the non-use of a polymerization initiator.
Japanese patent laid-open publication No. 2001-31709 discloses carrying out the bulk polymerization of a compound with a thiol group and a secondary hydroxyl group, and a compound with a secondary hydroxyl group without a thiol group as a catalyst thereof. Also, Japanese patent laid-open publication No. 2001-302705 discloses carrying out the bulk polymerization of a compound with both a thiol group and a carboxyl group in the substantial absence of an initiator.
However, in carrying out the above polymerization using a huge amount of thiol groups, the residue of thiol groups in syrup reduced the properties including storage stability of a final syrup product.
Japanese patent laid-open publication No. 2000-313704 discloses synthesizing an acrylate syrup with a conversion rate of 10-50%, in which 0.0001-0.5 parts by weight of an polymerization initiator with a 10-hr half-life at the temperature of less than 41° C. is used, the self-heat generation at reaction temperature of 20-80° C. is used, and the peak exothermic temperature of reactants reaches a range of 100-140° C. This polymerization method is based on the idea that the rapid increase in the concentration of radicals at the initial stage of reaction, resulted from the addition of an initiator which used to be decomposed at low temperature at a relatively high reaction temperature, causes the rapid increase of reaction speed, and as a result, the reactor temperature is increased, the half-life of the initiator drops rapidly and finally the depletion of initiator occurs resulting in stopping of reaction.
However, from the thorough examination, the present inventors concluded that this method needs a proper amount of an initiator to obtain a satisfactory result, which suggests a problem of high exothermic temperature. Thus, a highly sensitive cooling system is required during scale-up, nevertheless there is still a possibility of reaction runaway. In addition, the method which has the peak exothermic temperature of reactants ranging 100-140° C. is not applicable to low boiling point monomers such as ethyl acrylate (boiling point=99° C.) and butyl acrylate (boiling point=145° C.) and there is a problem of rapid spontaneous thermo polymerization by a high temperature unless the reactor is not rapidly cooled down at such high temperature. In the meantime, in production of high molecular weight acrylate syrup having more than a million of molecular weight, the conversion rate should be maintained as low as possible because of the steep increase in viscosity, but the method described in Japanese patent laid-open publication No. 2000-313704 is carried out without lowering the conversion rate, meaning that it has problems of unstable stirring and formation of gel due to the rapid increase of viscosity to produce a syrup having more than a million of molecular weight.